1. Field of the Invention
This invention relates to a heat-sensitive recording sheet having an excellent water-proofness.
2. Prior Art
A heat-sensitive recording sheet which utilizes a thermal color-forming reaction occurring between colorless or pale-colored chromogenic dyestuff and phenolic material, or organic acid is disclosed, for example, in the Japanese Patent Publication Nos. 4160/1968 and 14039/1970, and in the Japanese Laid-Open Patent Application No. 27736/1973, and is now widely applied for the practical use.
In general, a heat-sensitive recording sheet is produced by applying the sheet surface with a coating which is prepared by individually grinding and dispersing the colorless chromogenic dyestuff and color-developing material such as phenolic substance into fine particles, mixing the resulting dispersions with each other, and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. When this sheet is heated, the coating causes instantaneously a chemical reaction which forms a color. In this case, different bright colors can be advantageously formed, depending upon selection of specific colorless chromgenic dyestuff.
These heat-sensitive recording sheets have been found in a wide range of applications, including medical or industrial measurement recording instruments, terminal printers of computer and information communication systems, facsimile equipments, printers of electronic calculators, electronic balance, automatic ticket vending machines and so on. As the application use has become broader, a heat-sensitive recording paper is now used in various types of tickets such as railroad tickets and admission tickets, price cards attached to the commercial goods, as well as labels. On the other hand, the water-proofness of the recorded images has also come up to a problem depending on the cases. Meanwhile, the heat-sensitive recording paper is generally very much unstable to the water content or highly humidic condition and may reduce the density of the developed color images or even eliminate the color images due to the deposition of the water dropletes. Such an unstableness against the water is a serious problem in promoting the application of the heat-sensitive recording paper.